This invention relates to a thermoelectric refrigeration material suitable for thermoelectric elements having a thermoelectric conversion characteristic and performing a refrigerating operation when an electric current flows therethrough and a method of making the same.
The Peltier effect is usually utilized in refrigerators employing thermoelectric elements for performing the refrigeration. The thermoelectric element comprises two kinds of metals differing from each other in the electric characteristics or semiconductors bonded together. A current is caused to flow across both ends of the thermoelectric element such that heat absorbing or heating phenomena are caused. A refrigerating action can be obtained from the heat absorbing phenomena.
Various materials have been found as those with the Peltier effect. Semiconductive materials such as a system of bismuth (Bi) and antimony (Sb) or a system of bismuth and tellurium (Te) show an eminent effect as the thermoelectric refrigeration material. These semiconductive materials have been formed in a metallurgical thermally equilibrium condition and p-type and n-type conductors are combined to form the thermoelectric element.
It is desirable that the thermoelectric refrigeration material employed to form the thermoelectric element should have high thermoelectric conversion efficiency. However, the thermoelectric conversion efficiencies of the hitherto found thermoelectric materials are so low that they are not effective means for performing refrigeration in household refrigerators or the like having a relatively large capacity. Consequently, the thermoelectric refrigeration materials have conventionally had limited usage such as infrared detectors or semiconductor laser diodes in which a local refrigeration is performed or the case of temperature control when objects refrigerated have small volume or capacity.
A figure of merit Z presented by the following equation is generally used to evaluate the thermoelectric conversion performance of the above-described thermoelectric refrigeration materials: EQU Z=S.sup.2..sigma./K (1)
where S is the thermoelectric power, .sigma. is an electric conductivity, and K is thermal conductivity. The thermoelectric conversion efficiency becomes higher as the figure of merit Z is increased. To increase the figure of merit Z, it is desirable to increase the thermoelectric power S and the electric conductivity .sigma. and to reduce the thermal conductivity K, as obvious from the equation (1).
In the prior art, however, many trials to improve the performance of the thermoelectric refrigeration materials have been directed mainly to obtaining a material having a large value of the thermoelectric power S depending upon properties peculiar thereto, thereby improving the figure of merit Z of the thermoelectric refrigeration material. These conventional trials have not improved the figure of merit Z as much as expected.
The inventors have considered that reducing the denominator of the equation (1) or the thermal conductivity K should be expedient so that the figure of merit Z is improved to a large extent. However, it has been difficult to control the thermal conductivity in accordance with the conventional metallurgical method, and the control of the thermal conductivity have not been successfully performed hitherto.